1. Field of the Invention
This invention relates to the field of protective devices for electric machinery, in general, and in particular, to protective devices which detect over-load and light-load conditions in electric machinery.
2. Prior Art
Many kinds of machinery, driven by electric motors or other kinds of electrical drive mechanisms, usually require over-load and light-load protection. In the case of over-load conditions, excessive heating and current draw can result in destruction of the machinery, and in some cases, a general fire hazard. Under light-load conditions, such machines will tend to operate at unsafe higher speeds or rates of rotation, engendering thermal damage and structural failure. In either case, the risk of explosion or other catastrophic failure is a distinct possiblity.
A good example is that of a ram which is operated under a no-load condition, that is, at a decreased water level. The pump itself is not only damaged, but the cooling unit to which the water is otherwise fed from the pump is unduly heated as well. Another example is that of an operating chain apparatus in a sludge water processing equipment. Such chains are immersed in the sludge water and are particularly subject to corrosion. When such chains snap, as expected from time to time, the breaking of the chain should be sensed in order to sound an alarm.
In the conventional type of protective devices for electric machinery known heretofore, for use in protecting against the effects of over-load and light-load conditions, a contact in the control unit of an electric motor is closed simultaneously with start-up of the motor, starting a timer, in order to prevent the protective device from operating during the start-up period and the opening of a contact in the control unit is utilized in order to distinguish the no-load operation of a motor from shut-down thereof. In such systems, the protective device must be designed and installed fully in relationship with said contacts in the control unit for the electric motor of the particular machinery.